Mechanoluminescence material and process for producing the same

ABSTRACT

A mechanoluminescence material comprising a matrix of composite metal oxide containing strontium and aluminum, represented by the general formula SrM 1 Al 6 O 11  (wherein M 1  is an alkaline earth metal) or SrM 2 Al 3 O 7  (wherein M 2  is a rare earth metal), and further comprising, as luminescence centers, a metal selected from among rare earth metals and transition metals capable of emitting light when a carrier having been excited by mechanical energy returns to its ground state.

TECHNOLOGICAL FIELD

The present invention relates to a novel mechanoluminescence materialcapable of emitting light by receiving an external mechanical force aswell as to a method for the preparation thereof.

BACKGROUND TECHNOLOGY

The phenomenon that a substance emits visible light or light in thevicinity of visible light at a low temperature, e.g., room temperature,when it receives a stimulation from outside is heretofore well known asa so-called phenomenon of fluorescence. Such a substance to cause thephenomenon of fluorescence or, namely, a phosphor, is employed forilluminating lamps such as fluorescent lamps and the like and fordisplays such as CRTs (cathode-ray tubes) or, namely, so-called Brauntubes, and the like.

The external stimulation to cause the phenomenon of fluorescence isusually given by ultraviolet light, electron beams, radiations such asX-rays, electric fields, chemical reactions and others but noinvestigations have yet been made for a material capable of emittinglight when stimulated by a mechanical external force and the like.

The inventors previously made proposals for a high-brightness stresslight-emitting material consisting of a substance formed from analuminate having a non-stoichiometric composition and having latticedefects emitting light when carriers excited by mechanical energy returnto the ground state or a substance containing, in the above matrixsubstance, rare earth metal ions or transition metal ions as the centerions of the center of luminescence (official publication of JapanesePatent Kokai No. 2001-49251) and a light-emitting material in which thematrix material was Y₂SiO₅, Ba₃MgSiO₆ or BaSi₂O₅ (official publicationof Japanese Patent Kokai No. 2000-313878). These luminescence materialsare not suitable for practical applications because of their stillinsufficient luminescence intensity and, in addition, the limitation ofthe range of the matrix materials to be used so that the applicationfield is necessarily limited.

DISCLOSURE OF THE INVENTION

The present invention has been completed under these circumstances withan object to accomplish, by using a matrix material different from thosein the conventional luminescence materials, an increase in theluminescence intensity and expansion of the application fields.

The inventors have continued extensive investigations in order todevelop a novel mechanoluminescence material with the matrix materialdifferent from those in the prior art mechanoluminescence materials and,as a result, have arrived at a discovery that a novelmechanoluminescence material can be obtained including several materialsexhibiting a very high luminescence intensity, by using certaincomposite metal oxides containing strontium and aluminum as the matrixmaterial doped with metal ions of a specific kind as the center ofluminescence leading to completion of the present invention on the baseof this discovery.

Namely, the present invention provides a mechanoluminescence materialcharacterized in that the matrix material is a composite metal oxidecontaining strontium and aluminum as represented by the general formulaSrM¹Al₆O₁₁  (I)

(M¹ in the formula is an alkaline earth metal including strontium) orSrM²Al₃O₇  (II)

(M² in the formula is a rare earth metal)

and the center of luminescence is a metal ion selected from a rare earthmetal or a transition metal capable of emitting light when carriersexcited by mechanical energy return to the ground state as well as amethod for the preparation of the mechanoluminescence material whichcomprises the step in which powders of salts or oxides of the respectiveingredient metals corresponding to the composite metal oxide containingstrontium and aluminum as represented by the above-given general formula(I) or (II) is admixed with a salt or oxide of specific rare earthmetals or transition metals capable of emitting light when carriersexcited by mechanical energy and a flux return to the ground state in aproportion to make up 0.0001 to 20% by moles calculated for the metalatoms and blended; and the step of firing the thus obtained mixture at400 to 1800° C. in a reducing atmosphere to effect doping of the centerof luminescence.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a graph showing the luminescence intensity as a function oflapsed time when a mechanical acting force is applied to amechanoluminescence material as an example in Example 1 of the presentinvention.

FIG. 2 is a graph showing the relationship between the luminescenceintensity and the load applied to a luminescence material as an examplein Example 1 of the present invention.

BEST MODE FOR CARRYING OUT INVENTION

In the following, the present invention is described in detail.

In the mechanoluminescence material of the present invention, acomposite metal oxide containing strontium and aluminum forming thematrix material has a composition as represented by the above-givengeneral formula (I) or (II). The alkaline earth metal as M¹ in thegeneral formula (I) is preferably Ba, Ca, Sr or Mg.

Further, examples of the rare earth metal as M² include La, Y, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu of which La and Y areparticularly preferable. M¹ and M² in these general formulas (I) and(II) can be a single kind or a combination of two kinds or more.

Sr₂Al₆O₁₁, SrMgAl₆O₁₁, SrCaAl₆O₁₁ and SrBaAl₆O₁₁ are preferable amongthe composite metal oxides as represented by the general formula (I) andSrLaAl₃O₇, SrCeAl₃O₇, SrPrAl₃O₇, SrNdAl₃O₇, SrSmAl₃O₇, SrGdAl₃O₇ andSrYAl₃O₇ are preferable among the composite metal oxides as representedby the general formula (II) since a high light-emitting intensity can beefficiently obtained. Among the above-mentioned composite metal oxidesas represented by the general formula (II), SrLaAl₃O₇ and SrYAl₃O₇ areparticularly preferable.

In the next place, the specified rare earth metal or transition metalions can be any metal ions capable of emitting light when carriersexcited by mechanical energy return to the ground state and are notparticularly limited.

The rare earth metal can be exemplified, for example, by Sc, Y, La, Ce,Pr, Nd, (Pm), Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, and thetransition metal other than rare earth metals can be exemplified, forexample, by Ti, Zr, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Nb, Mo, Ta, W and thelike.

These metals can serve singly for the center of luminescence or canserve for the center of luminescence as a combination of two kinds ormore.

The metal which is optimum for the center of luminescence differsdepending on the crystalline structure of the matrix material. The mostpreferable are, for example, Eu when the matrix material is Sr₂Al₆O₁₁,SrMgAl₆O₁₁, SrLaAl₃O₇ or SrYAl₃O₇ and Ce when it is SrCaAl₆O₁₁.

It is necessary that the matrix material contains a rare earth metal ora transition metal in a proportion to make up 0.0001 to 20% by molescalculated for the metal atoms. When this amount is smaller than 0.0001%by moles, no sufficient improvement can be obtained in the luminescenceintensity while, when in excess over 20% by moles, the crystallinestructure of the matrix material cannot be retained resulting in adecrease in the luminescence efficiency. A preferable content is in therange from 0.1 to 5.0% by moles.

The mechanoluminescence material of the present invention can beprepared, for example, by blending powders of salts or oxides of therespective ingredients capable of forming the composite metal oxiderepresented by the general formula (I) or (II) in a proportioncorresponding to the respective composition of the general formulas,admixing, together with 5 to 20% by mass of a flux such as boric acid, asalt or oxide of the rare earth metal or transition metal in 0.0001 to20% by moles or, preferably, in 0.1 to 5.0% by moles calculated for themetallic element and blending followed by firing at a temperature of 400to 1800° C. or, preferably, 800 to 1500° C. in a reducing atmosphere soas to effect doping of the center of luminescence. The reducingatmosphere in this case includes an atmosphere of a gas mixture ofhydrogen gas and an inert gas such as, for example, helium, argon andneon. The firing time is usually in the range from 1 to 10 hours.

The salts or oxides of the respective ingredient metals capable offorming the composite metal oxide, in this case, are exemplified, forexample, by carbonates, nitrates, chlorides, acetates and the like ofSr, Ca, Ba, Mg, La, Y and the like as well as oxides of these metals orAl. Salts of rare earth metals or transition metals as the center ofluminescence are exemplified, for example, by nitrates and chlorides.

The luminescence intensity of the mechanoluminescence material of thepresent invention obtained in this way is, as a trend, usually higherwith a larger mechanical acting force applied though dependent on themechanical acting force as the excitation source. Accordingly, it ispossible to obtain the mechanical acting force added to the luminescencematerial by measuring the luminescence intensity thereby to affordnon-contacting detection of the stress loaded on the material.

With regard to the mechanoluminescence material of the presentinvention, a laminated material can be prepared by forming a coatingfilm containing the same on the surface of a base material.

Though without particular limitations with respect to this basematerial, the material thereof includes, for example, quartz, silicon,graphite, plastics, metals, cement and the like.

In the following, the present invention is described in more detail byway of examples although the present invention is never limited by theseexamples.

EXAMPLE 1.

A mechanoluminescence material was prepared by blending SrCO₃, MgCO₃ andAl₂O₃ each in a powder form having an average particle diameter of about10 μm in a proportion corresponding to SrMgAl₆O₁₁ as the matrixmaterial, thoroughly blending with further addition of 20% by moles of aboric acid powder (average particle diameter 10 μm) as a flux and 0.5%by moles (calculated for the metallic element) of a Eu₂O₃ powder to givethe center of luminescence and firing the thus obtained blend at 1300°C. for 4 hours in an atmosphere of argon containing 5% by volume ofhydrogen.

In the next place, a sample was prepared by pelletizing thismechanoluminescence material with an epoxy resin as a binder.

By using a vise, a mechanical action of 150 N was applied to this sampleand the change with time in the luminescence intensity there is shown asa graph in FIG. 1. This sample emitted green light which was strongenough as to be clearly recognizable with naked eyes. The highestluminescence intensity (cps) there is shown in Table 1.

In the next place, observation was made for the condition ofluminescence of this sample under varied loads applied thereto toexamine the stress dependency. The results are shown in FIG. 2 as agraph.

As is understood from this figure, the luminescence intensity dependedon the stress and was increased with an increase in the load. It isunderstood from this fact that the value of the stress applied can beestimated by measuring the luminescence intensity.

COMPARATIVE EXAMPLE

A mechanoluminescence material was prepared under just the sameconditions as in Example 1 excepting for the omission of the Eu₂O₃powder and the highest luminescence intensity was determined in the samemanner as in Example 1. The result is shown in Table 1.

EXAMPLES 2 TO 4.

By using the matrix materials indicated in Table 1, mechanoluminescencematerials with Eu to serve as the center were prepared in the samemanner as in Example 1. The highest luminescence intensities thereofwere determined and the results are shown in Table 1.

TABLE 1 Matrix Center of luminescence material luminescence intensity(cps) Example 1 SrMgAl₆O₁₁ Eu 24990 Example 2 Sr₂Al₆O₁₁ Eu 9787 Example3 SrLaAl₃O₇ Eu 28694 Example 4 SrYAl₃O₇ Eu 4611 Comparative SrMgAl₆O₁₁None 91 Example

As is understood from this table, the luminescence intensity isremarkably increased by doping with a metal for the center ofluminescence.

INDUSTRIAL UTILIZABILITY

According to the present invention, a novel mechanoluminescence materialcan be obtained which efficiently emits luminescence by a mechanicalouter force such as a frictional force, shearing force, impact force,compressive force and others. Direct conversion of the above-mentionedmechanical outer forces into light can be accomplished by theluminescence of the material per se under action thereof so that a widerange of applications can be expected including a possibility ofutilization as a quite novel device of luminescence and so on.

1. A mechanoluminescence material wherein the matrix material is acomposite metal oxide containing strontium and aluminum as representedby the formula SrM²Al₃O₇ (M² in the formula is a rare earth metal) andthe center of luminescence is europium.
 2. The mechanoluminescencematerial described in claim 1 in which the composite metal oxidecontaining strontium and aluminum is SrLaAl₃O₇ or SrYAl₃O₇.
 3. A methodfor the preparation of a mechanoluminescence material wherein powders ofsalts or oxides of the respective ingredient metals corresponding to acomposite metal oxide containing strontium and aluminum as representedby the formula SrM²Al₃O₇ (M₂ in the formula is a rare earth metal) areadmixed with a salt or oxide of europium in a proportion to make up0.0001 to 20% by moles calculated for the europium atoms and M² rareearth metal atoms and blended followed by firing at 400 to 1800° C. in areducing atmosphere to effect doping of the center of luminescence.